The invention relates to optical waveguides comprising photosensitive glass, and to optical waveguide devices containing such glass, such as optical fiber gratings, optical fiber dispersion compensators, optical fiber sensors, optical fiber lasers and planar waveguide devices.
Photosensitivity of glass materials (i.e. change in refractive index by exposure to optical radiation) is a fundamental property for the realization of many optical devices. Typical glasses used for these applications are photosensitive to ultraviolet (UV) light. For example, Bragg gratings in optical fibers and planar waveguides have attracted much attention since their first demonstration. Bragg gratings are obtained by exposure of a waveguide made of photosensitive glass to a periodic light pattern. Such devices have many applications, such as sensors, dispersion compensators and laser mirrors.
The definition of channel waveguides using UV writing on glass substrates, such as silica on silicon, is also becoming increasingly important for the realization of multifunctional integrated optical components.
Telecommunications optical fibers, i.e. SiO2 fibers with a low content of GeO2 (xcx9c3%), show only relatively small refractive index changes when exposed to UV radiation. In addition, the UV photoinduced index change in standard germanosilicate waveguides is not sufficient for writing complex structures with a high degree of integration. With known photosensitive glasses, it is thus necessary to increase the photosensitivity of optical fibers or planar waveguides through post-fabrication methods and co-doping. Typical post-fabrication methods are hydrogen/deuterium loading and flame brushing. Typical co-dopants are B2O3, SnO2 and rare earth elements. SnO2 is used mainly as a codopant in germanosilicate, phosphosilicate and aluminosilicate glasses [see references 1,2,3,4]. Optical fiber with numerical apertures (NA) between 0.23 and 0.29 and refractive index modulations of xcx9c10xe2x88x923 can be produced in this way.
Recently, optical fibers made from SnO2:SiO2 have been investigated [5]. Tin doped glass is also mentioned by other workers [6-8]). In reference [5], it has been shown that small concentrations (xcx9c0.15 mol %) of SnO2 in a silica network give permanent refractive index changes with a high degree of photorefractivity. Under similar UV irradiation conditions, the degree of photorefractivity is nearly two orders of magnitude higher than the concentrations of GeO2 required in SiO2:GeO2 fibers.
Compared to the above-mentioned post-fabrication and co-doping techniques, the use of SnO2 has several potential attractions. It should keep the absorption low in the third telecom window at 1.5 xcexcm. Processing is potentially less time consuming and potentially cheaper.
However, in practice, preparation of SnO2:SiO2 glasses is limited by the low solubility of Sn which crystallizes out of SnO2 at concentrations greater than about 1 mol %. Inclusion of SnO2 at higher concentration leads to a crystalline material which is not viable for realization of devices with low optical loss. Although non-crystalline photosensitive silica glass with SnO2 concentrations of close to 1 mol % could potentially lead to the production of practical devices, the 1% solubility limit represents a major practical limitation, which limits the amount of photosensitivity (refractive index modulation) and the control of NA.
Another problem with the production of SnO2 doped or co-doped fiber via modified chemical vapor deposition (MCVD) or solution doping techniques is related to the high volatility of SnO2 at the temperature required for preform collapse. This problem effectively prevents high SnO2 concentrations being incorporated into a fiber. An analogous problem can be expected to arise for other glass preparation processes.
It is therefore an aim of the invention to provide a photosensitive glass for fabricating fiber-optic and planar waveguides that allows tin oxide to be incorporated in a silica matrix at concentrations above its normal solubility limit of 1 mol %, and also that reduces the effects of high volatility during preform collapse or other glass preparation process.
It has been discovered that inclusion of a Group I element such as sodium in a tin-doped silica glass matrix provides a glass with highly desirable properties for use in waveguides, either made from optical fibers with the glass forming the core, or as part of a planar waveguide structure.
Accordingly, the invention provides an optical waveguide having a waveguiding channel of photosensitive glass with a modified refractive index optically written therein, wherein the photosensitive glass comprises an oxide of sodium or another Group I element such as lithium or potassium in a matrix of silicon and tin oxides. The optical waveguide may be an optical fiber, with the waveguiding channel being formed by a core of the optical fiber comprising the photosensitive glass. The optical waveguide may also be a planar waveguide device comprising a layer of the photosensitive glass with a waveguiding channel optically written into the layer. Integrated planar waveguide structures may thus be fabricated by writing a channel network into the photosensitive glass layer.
Inclusion of an oxide of a Group I element such as Na, Li or K can be used to increase the solubility of tin in the oxide up to 20 times above the 1 mol % limit of tin in the binary compound SiO2:SnO2 thereby allowing larger refractive index modulations to be achieved. In an embodiment of the invention, the glass comprises the oxides SiO2, SnO2 and Na2O. The inclusion of sodium oxide with silica and tin oxide has been shown to produce a photosensitive glass with a highly desirable combination of properties.
A further advantage of inclusion of Na is that the effects related to high volatility of Sn during preform collapse are reduced.
Moreover, refractive index modulations optically written into the SiO2:SnO2:Na2O glass have been shown to have remarkable temperature stability, much superior to conventional SiO2:GeO2 glasses. It has been shown that the temperature stability is at least as good as that of the binary photosensitive glass SiO2:SnO2. Sodium oxide incorporation can thus be used to increase Sn concentration to increase intrinsic photosensitivity, while simultaneously providing a glass in which optically written refractive index modulations are highly stable.
The introduction of sodium oxide also does not cause any significant change in the background refractive index. This contrasts with other dopants (e.g. Ge or P) which might be suitable for increasing the solubility limit of Sn but which increase the background refractive index of the silicate glass to a much greater degree. For example, in conventional germanosilicate glass the background refractive index increases by about 0.002 per mol % of GeO2 in SiO2 (see reference 9). On the other hand, for sodium the increase is about 5 times less, being about 0.0004 per mol % for Na2O in SiO2. This is an important advantage for sodium, because, if the refractive index is too high, it is difficult or impossible to manufacture optical fibers or planar waveguides that are compatible with current telecom fibers.
The photosensitive glass preferably comprises between about 1-20 mole % SnO2 and 1-60 mole % R2O (where R=Na, K or Li) to lie within the solubility limit in a generally silica matrix. The photosensitive glass may comprise at least one of: 2, 5 or 10 mole % SnO2 and/or at least one of: 2, 5, 10, 20, 30 or 40 mole % R2O.
It will also be understood that the combination of tin and Group I oxides can be applied not only to pure silica glass, but also to silica doped with other elements, e.g. to germanosilicate glass, phosphosilicate glass or aluminosilicate glass, or combinations thereof. Additional rare earth dopants such as Er, Nd or Yb may also be included to provide gain. References to silica glass and a silica matrix are therefore to be construed as covering silica doped with other elements such as germanium, phosphorous or aluminum. Similarly references to a ternary compound are to be construed as inclusive of quaternary and higher order compounds.
The photosensitive glass may be used for many device applications. Specifically, an optical fiber can be provided that has a core made of the photosensitive glass. A planar waveguide comprising the photosensitive glass can also be provided. There can also be provided an optical device having a gain medium comprising the photosensitive glass, for example a laser, optical amplifier or light emitter. To provide gain in the photosensitive glass, suitable dopants such as the rare earth elements Er, Nd, Yb may be included. Moreover, a grating structure may be written into the photosensitive glass, as is described further below in the specific description.
Because of the remarkable stability of refractive index modulations optically written into the glass, devices made of the glass are more robust to intrinsic absorption, multi-photon absorption, high temperatures and other related effects which all tend to erase optically-written refractive index modulations.
Consequently, the glass is especially attractive for applications such as high power lasers and amplifiers where residual absorption effects or multiphoton absorption are typically large and where significant temperature increases may arise during operation. Since the refractive index modulations can withstand such high operating temperatures, devices incorporating the glass can be run hotter and at higher intensities. In this way cooling requirements are less stringent and devices can be run at higher output levels. More generally, the high temperature stability is indicative of a high characteristic activation energy for the photoinduced refractive index changes, which are thought to originate from electron trapping effects. Consequently, even if device operating temperatures are kept low by cooling or pulsed operation, high power devices incorporating a modulated refractive index profile optically written into the glass are expected to demonstrate enhanced stability. For example, device reliability and operating life are expected to be improved.
According to a further aspect of the invention there is provided an optical waveguide device comprising: an optical waveguide formed at least in part of photosensitive glass, wherein the photosensitive glass is doped with tin as a photosensitizing dopant; and a refractive index variation optically impressed on the photosensitive glass of the optical waveguide; wherein the photosensitive glass is doped additionally with a Group I element as a dopant for increasing solubility of tin in the glass.
According to another aspect of the invention there is provided a method of fabricating an optical waveguide, the method comprising: providing a photosensitive glass doped with tin as a photosensitizing dopant and a Group I element as a dopant for increasing solubility of tin in the photosensitive glass; and exposing regions of the photosensitive glass to refractive index change inducing optical radiation, the exposed regions providing a light-guiding core of raised refractive index within the photosensitive glass.
According to a still further aspect of the invention there is provided a process for increasing the sensitivity of optical glass to light-induced refractive index variations, the process comprising: doping the optical glass with tin as a photosensitizing dopant in combination with a Group I element as a further dopant for incorporating increased amounts of tin in the optical glass, thereby to increase the sensitivity of the optical glass to light induced refractive index variation; and exposing regions of the optical glass to varying optical radiation to modulate the refractive index of the optical glass.